1. Field of the Invention
The present invention relates generally to the field of data storage, and more particularly to the facilitating readout from a data storage device.
2. Description of the Related Art
Storage media for computers and other types of electronic devices include volatile memory and non-volatile memory. Volatile memory loses its contents when power is no longer supplied to the memory, whereas non-volatile memory maintains its contents even when power is not supplied to the memory. The most common type of volatile memory is dynamic random-access memory (DRAM), commonly available as and implemented as an integrated circuit (IC). Non-volatile memory has been available in the form of magnetic and optical media, including hard disk drives, floppy disks, compact disc read-only memories (CD-ROMs), CD re-writable (CD-RW) discs, and digital versatile discs (DVDs), among others. Historically, non-volatile memory implemented as an IC was primarily available as ROM that was not re-recordable, such as hard-wired ROM and programmable ROM (PROM). More recently, IC non-volatile memory has become available as various types of flash memory, which is more technically known as electrically erasable PROM (EEPROM).
Storage density of the storage media employed in computing devices is ever increasing. One available tool known today that provides enhanced storage density and may be scaled to ever smaller sizes, such as down to the nanometer scale, is a nanometer probe tip. Nanometer probe tips are used in atomic force microscopes (AFM) and scanning tunneling microscopes (STM) for imaging and structuring down to the atomic scale. The simple tip is a very reliable tool that provides enhanced local confinement of interaction.
In recent years, AFM thermo-mechanical recording in polymer storage media has undergone extensive modifications mainly with respect to the integration of sensors and heaters designed to enhance simplicity and to increase data rate and storage density. Using heated cantilevers, thermo-mechanical recording at 400 Gb/in2 storage density and data rates of a few Mb/s for reading and 100 kb/s for writing have been demonstrated.
Such prior thermo-mechanical writing applies a local force to a polymer layer using a cantilever/tip and softens the polymer layer using local heating. Application of sufficient heat forms an indentation in the storage medium, forming a written bit. The same tip can read the written bit by the deflection of the cantilever when moved into the indentation, in combination with the electrical resistance of a sensing circuit based on cantilever movement.
While writing data or bits, the heat transfer from the tip to the polymer through the small contact area is initially very poor and improves as the contact area increases. The tip is heated to a relatively high temperature to initiate the melting process. Once melting has commenced, the system presses the tip into the polymer, increasing heat transfer to the polymer and the volume of melted polymer, and hence increasing bit size. After melting has started and the contact area has increased, the heating power available for generating indentations increases by at least ten times to become 2% or more of the total heating power, depending on the design. In order to provide a complete data storage method, a data read process should provide an adequate Signal-to-Noise Ratio (SNR) at an acceptable data rate
One method for reading currently available depends on the modulation of the gap between a warm (non-writing) cantilever and the medium. Gap modulation results from the tip following medium topography introduced by the foregoing writing process. The gap modulation generates a synchronous modulation in the cantilever temperature through a variation in thermal flux between cantilever and medium. The temperature coefficient of resistivity of the heater or other temperature sensing element on the cantilever translates this temperature variation into a resistance variation, which is sensed by appropriate electronics as the output signal. The bandwidth or data rate for this read method is fundamentally limited by the thermal time constant of the heater/cantilever.
In addition to an SNR problem with the aforementioned readback scheme, the scheme also has difficulty addressing large arrays of tightly packed probes. Probes may be positioned parallel to one another and/or in relatively close proximity, but design advantages may be realized by utilizing alternative readback schemes whose bandwidth is not limited by thermal time constants and which minimize the area required for the read/write/erase sense and control electronics, irrespective of the form or profile of the topographic bit.
It would be advantageous to provide a design that reads these bits at a relatively high data rate with a similar or improved SNR over what has been previously available and avoids the problems associated with previous designs.